1. Field of the Invention
The present invention relates in general to control systems of an automotive automatic transmission, and more particularly to the control systems of a type which can engage a friction element to bring about an engine brake when, with the transmission assuming a given forward speed of a so-called Over-drive possible D-range, a throttle opening becomes reduced to a certain small degree due to OFF-operation of an accelerator.
2. Description of the Prior Art
For clarifying the task of the present invention, one known control system of an automotive automatic transmission will be briefly described with reference to FIG. 6 of the accompanying drawings. The transmission of the drawing is described in the manual (A261C07) titled "MAINTENANCE FOR FULL-RANGE .ELECTRONICALLY CONTROLLED AUTOMATIC TRANSMISSION (RE4R01A type)" issued from NISSAN MOTOR CO., LTD in March, 1987.
In the known control system, in a forward drive range with an over-drive switch being ON, engine brake does not operate irrespective of the degree of a throttle opening, while, in a forward drive range with the over-drive switch being OFF, the engine brake operates in the second or third forward speed when the throttle opening becomes small. Either one of these two forward drive ranges is selected at will by manipulating the over-drive switch (OD-SW) which is usually mounted on a shift lever.
For ease of description, the forward drive range with the over-drive switch being ON will be referred to as "Over-drive possible D-range" wherein the transmission can assume a lock-up ON condition, and the forward drive range with the over-drive switch being OFF will be referred to as "Over-drive impossible D-range" wherein the transmission can not assume the lock-up ON condition.
As is described in page I-65 of the manual, in the above-mentioned known control system of the automatic transmission, in case of the Over-drive possible D-range, an overrun clutch (OR/C) 18 (which is a friction element for achieving engine brake) is kept disengaged even when, with the transmission assuming a lock-up ON condition, the driver releases an accelerator or effects an OFF operation of the accelerator.
That is, in the known control system, in the Over-drive possible D-range, the same control map-is used for both a case wherein the lock-up ON condition is established and a case wherein the lock-up ON condition is not established. Accordingly, even when, with the transmission assuming the third forward speed in the Over-drive possible D-range, the throttle opening is reduced to a certain degree due to OFF-operation of the accelerator, the overrun clutch (OR/C) 18 is not engaged. Thus, upon the OFF-operation of the accelerator, a forward one-way clutch (F/OC) 21 is forced to run idle, so that a counter driving force is not transmitted to the engine and thus an engine brake is not applied to the output shaft 12. This phenomenon brings about a sudden drop of the engine speed.
Thus, when, for improving a fuel economy, a fuel cut control has been set to control an associated engine, the control would fail to operate normally. That is, normally, the fuel cut action takes place when, with the engine speed exceeding a predetermined degree, the throttle opening is reduced to a certain level. However, due to dropping of the engine speed, the fuel cut action can not take place even when the throttle opening is reduced.